In recent years, regarding a liquid fuel such as gasoline and gas oil, the control of a sulfur content and an aromatic hydrocarbon content has become rapidly stringent. Therefore, it has become essential to produce a clean liquid fuel having low sulfur and aromatic hydrocarbon contents and being so-called friendly to the environment. Examples of the method for producing such a clean fuel include a method of utilizing so-called Fischer-Tropsch synthesis reaction (hereinbelow, may also be referred to as “FT synthesis reaction”) for reducing carbon monoxide with hydrogen (hereinbelow, may also be referred to as “FT synthesis method”). By the FT synthesis method, not only can be produced a liquid fuel base stock rich in a paraffin hydrocarbon and containing no sulfur, but also can be produced a wax (hereinbelow, may also be referred to as “FT wax”) simultaneously. Then, the FT wax can be converted into a middle distillate (a fuel base stock such as kerosene and gas oil) by hydrocracking.
As the catalyst used for the FT synthesis method (hereinbelow, may also be referred to as “FT synthesis catalyst”), a catalyst in which an active metal such as iron, cobalt, and ruthenium is supported on a porous inorganic carrier such as silica and alumina is generally used (for example, see Patent Literature 1). In addition, with respect to the FT synthesis catalyst, it is reported that by using the above active metal in combination with a second component metal compound, the catalyst performance is enhanced (for example, see Patent Literatures 2 and 3). Examples of the second component metal include sodium, magnesium, lithium, zirconium, and hafnium, which are used as required in accordance with the intended use such as enhancing the inversion rate of carbon monoxide or increasing the chain growth probability, an indicator of a wax generated amount.
The FT synthesis catalyst is generally produced by supporting a metal compound containing an active metal component on a carrier, in which a metal component as a second component is supported on a porous inorganic oxide, and calcining this carrier product to convert the active metal component into an oxide. Further, by reducing the above-mentioned catalyst, the active metal component is converted from the oxide into a metal, thereby obtaining an FT synthesis catalyst (hereinbelow, may also be referred to as “activated FT synthesis catalyst”) having high activity, and this FT synthesis catalyst is used in the FT synthesis reaction. In a known method for activating a cobalt catalyst, which is a typical FT synthesis catalyst, that is, in reduction of a cobalt catalyst in a stream of a hydrogen gas or a gas containing hydrogen, Co3O4, which is a cobalt species generated on the carrier by calcining, is finally reduced to Co (metal Co) via CoO. In addition, in the case of a ruthenium catalyst, RuO is reduced to Ru.
Note that generally, an FT synthesis catalyst is subjected to a reduction treatment in an apparatus attached to a catalyst producing facility, and thereafter, the outer surface of the catalyst is coated with a wax or the like, or an FT synthesis catalyst is ordinarily subjected to a stabilization treatment by lightly oxidizing the outer surface thereof and then transported to a hydrocarbon producing facility in which the FT synthesis method is performed, in order not to cause a decrease in activity of the catalyst by a contact of the catalyst with air during the transport or the like of the catalyst. In this description, the term “activated FT synthesis catalyst” encompasses catalysts subjected to the above-mentioned stabilization treatment.